Mica condenser with hard-metal terminal foils and lead-tin foil connections to the electrodes



Jan. 1l, 1966 J. F. MCHUGH MICA CONDENSER WITH HARD-METAL TERMINAL FOILSAND LEAD-TIN FOIL CONNECTIONS TO THE ELECTRODES Original Filed Jan. l0,1961 INVENTOR. JAMES F. MCHUGH ATTORNEY United States Patent tiice3,229,173 Patented Jan. 1l, 1966 MICA CONDENSER WITH HARD-METAL TERMI-NAL FOILS AND LEAD-TIN FOIL CUNNEC- TIONS TO THE ELECTRODES James F.McHugh, Cranston, RJ., assigner to Cornell- Dubilier ElectricCorporation, a corporation of Delaware Original application Jan. 10,1961, Ser. No. 81,891, now Patent No. 3,151,382, dated Oct. 6, 1964.Divided and this application Feb. 6, 1964, Ser. No. 350,935

5 Claims. (Cl. 317-258) This is a division of my copending applicationSerial No. 81,891 tiled January l0, 1961, now Patent No. 3,151,382.

The present invention relates to mica condensers.

An object of this invention is to devise a novel and compact form ofmica condenser construction. Another object resides in features ofconstruction of small stackedmica condensers that are more economical toproduce and more compact for a given electrical size than comparableprior art types of mica condensers having metal end clips. A collateralobject of the invention resides in the provision of a novel condenserconstruction wherein electrodebearing micas can be stacked and processedas multiplecondenser units, which can then be cut apart into individualunits to which wire terminals can be soldered.

The various features of the invention may be practiced pursuant to theillustrative embodiment described in detail below. In that embodiment,electrodes of fired silver paint are formed as lms on the opposite facesof strips of mica. A number of these micas are stacked, and leadtinalloy foil is interposed between edge portions of confronting electrodeareas of the micas, alternately at one edge and at the opposite edge tofthe successive micas in the stack. In addition, a pair of foils of arelatively hard higher-melting-point metal are provided along the edgesof the sacked mica strips, including one such additional foil that isfolded around all of the mica elements and another that is folded aroundfall but one of the micas and is interposed between that one mica andthe remainder of the stack. The stacked rmica strips with the leadtinfoils and the relatively hard foils are then baked, to dispel surfacemoisture, and subjected to heavy flattening pressure below the meltingpoint of the lead-tin foils. This forms cold pressure welds between thelead-tin foils and the film electrodes and it also units the lead-tinfoils and the hard-metal foils where they overlie one another. Coldpressure welding, commonly called cold welding, is the process ofjoining metals under conditions where no liquid phase is produced orintnoduced into the weld at any stage and without reliance on heat toproduce fusion of the metals being joined. The pressed stack isimpregnated, pressed again to expel excess impregnating liquid, and theimpregnant is polymerized so that the stack of micas that is initiallyunited mechanically by the cold welds becomes a bonded unit. 'I'he pieceat this phase of the assembly is cut into tiny individual-condenserunits to which wire tenminals are secured by soldering. Subsequentapplication of a protective encapsulating coating completes eachcondenser.

The sequence of steps of cold-welding the film electrodes to the softfoils followed by impregnation and polymerization is of specialimportance. The initial step of cold-welding provides a series ofelectrical connections that are not impaired by the subsequentimpregnation and provides .a mechanical union between the micas, thusmaking impregnation of the stacked micas feasible. In turn, theimpregnation and polymerization greatly facilitates and promotessuccessful cutting of the tiny stacked units, and it imparts importantelectrical properties to the finished unit.

The inclusion of foils of hard strong metal (in contrast to the leadfoil that is sufficiently soft to fonm cold welds) is of considerableimportance. If only lead-tin foils were used, it would be difficult toprovide the oondensers with soldered wire terminals. The foils wouldmelt during soldering attempts, and are mechanically too weak to unitethe terminal wires to the mica stacks. In an effort to avoid this, metalend clips might be used or conceivably the wires might be wrapped aroundthe individual condenser stacked units. In either case the terminalstructure becomes bulky, prominently bulky in the case of tiny units.The increase in bulk is due not only to the mass of metal that forms theterminal, but this bulk is considerably enlarged by the encapsulatingcoating subsequently applied. By contrast, the present terminal foilsprovide strong land solderable tenminals without any appreciableincrease in bulk of the stacked condenser unit beyond that involved inthe unicas and the connecting lead-tin foils.

The use of these hard terminal foils is also of importance in themanufacturing operation. These terminal foils are incorporated in thecondenser units in the same operation as that which forms the stackedmicas into a unit. Moreover this is done at a time when each mica stackrepresents a number of condenser units, the solderable metal foils beingreadily sheared in the same operation that cuts through the micas andthe le-ad-tin foils. This procedure avoids handling of individual tinymetal terminals for assembly to individual stacked-mica condenser units,in a separate terminal-applying operation.

The nature of the invention, its various novel aspects and furtherobjects will be rmore fully apparent from the following detaileddescription of an illustrative embodiment of the invention, which isshown in the accompanying drawings. In those drawings:

FIGURE 1 is an enlarged plan view of an individual mica having bondedmetal-film electrodes;

FIGURE 2 is a greatly enlarged cross-section of stacked parts of a micacondenser, including three micas, of the form in FIGURE 1;

FIGURE 3 is a flow diagram of the operations performed in the novelmethod of manufacturing a mica condenser of novel construction, and

FIGURE 4 is an enlarged view of the novel condenser, as it appears justprior to lthe final encapsulation.

In FIGURE 1, a long and narrow strip 10 of mica is shown having a film12 of fired-on silver paint on its top face and a like pattern offired-on silver paint 14 on its opposite face. The xmetal lms 12 and 14include tongues 12a :and 14a that confront each other in a series ofareas there being eight such areas in this illustration. In practice theedges of tongues 12a and 14a are aligned, ideally, these tongues beingshown olf-set from each other for clearer illustration. Each lm 12 and14 has a marginal portion 12b and 14b which extends close to edges 10aand 10b, respectively. The lateral edges of the tongues are separated bygaps 13.

In an example, the micas may be only .O01 or .002 inch thick, 0.25 inchwide and 1.25 inches long for micas having twelve (rather than eight)confronting tongues 12a and 14a.

In FIGURE 2 a series of these micas bearing metal films are shownstacked. For convenience, the bottom mica is designated 10, the nextmica is designated and the third is designated 210. A dummy mica 18appears at the top, which does not have a lm electrode.

A number of foils 20, 22, 24, 26 and 28 are shown for providingconnection to areas 12b and 14b of rmicas 10, 110 and 210. of 0.0006inch lead-tin foil, and are relatively soft and malleable. A pair ofterminal foils 30 and 32 are shown, these being of relatively hard,solderable metal. These In an example, these connecting foils areterminal foils are of tinned copper approximately .O01 inch thick in anexample.

All of the foils are flat during the assembly operation, initially. Theelectrode-bearing micas, la dummy mica 18, and the foils are stackedwith each foil extending between the edge portions of confronting micasalternating between opposite edges of the stack. The foils are then bentupright while the micas are moderately pressed together, and finally thefoils are bent lover the top of the stack of micas.

The edges `of the micas lare all in alignment in the stack, and the filmelectrodes are applied unifonmly relative to the mica, and in this wayassurance is had that the confronting areas 12d and 14a of thesilver-film electrodes of successive micas overlie one another and thatthe spaces 13 between the confronting electrode areas are also t inalignment in the stack.

The stacked strips of micas with their metal films and interposed foilsfolded as shown in FIGURE 2 are next baked to drive-olf surface moistureon silver-film electrodes 12 and 14, at a temperature below themelting-point of the lead-in ffoil; and while thus heated, the stacksare subjected to intense pressure between flat opposed pressure members,as in a hydraulic press. This operation unites the stack as amechanically tight unit. It is also effective to make excellentelectrical connections from all the film electrodes to intense theterminal foils 30 and 32. The pressure is sufficient to form cold-weldsbetween the metal films 12 and 14 and the respective opposed foils 20,22, 24, 26 and 28; it cold-welds the terminal foils 30 and 32 to theconfronting foils 22, 26 and 28 respectively at areas facing iilmelectrodes 14 and 12 respectively; it cold-welds foils 24, 26, 28 and 32at a terminal region 34; and it cold-welds foils 20, 22 and 30 atanother terminal region 36. Considering this operation, it would bepossible to use a hard-metal foil 32 with an initially bonded -thicknessof suitably ductile metal replacing foils 26 and 28; and likewise foils22 and 30 might alternatively be bonded before being assembled. However,it has been found advantageous to assemble and cold-weld the filmelectrodes and the various foils as shown without requiring preparationof lspecial copper foils. An example of such special foil would be .O01inch copper foil bearing lead-tin bonded layers of .0006 inch thickness,to substitute for the separate foils of the illustrative dimensionsgiven above.

The stacking and pressing operations described are the lirst two stepsdiagrammatically illustrated in FIGURE 3. The unit thus produced is nextimpregnated with an insulating liquid, polyvinylformalacetyl resin forexample, by usual vacuum-impregnation techniques. The stacked micas,bonded by the foils, can be handled in bulk during this operation.

During the impregnating operation, the impregnant not only lls the voidsbetween the micas, but the impregnant also coats the entire mica stackincluding the areas where terminal wires are later to be soldered. Theimpregnant should be one selected Ito be compatible with this solderingoperation if scraping is to be avoided preparatory to the subsequentsoldering operation. Polyvinylformalacetyl resin is one example of animpregnant that satisfies this soldering condition, but others are knownthat meet this requirement.

The impregnated units are then spread on a flat surface and pressed,large numbers of these units being pressed concurrently for expellingsurplus impregnant. Moderate heating then effects polymerization,preferably controlled at this time to effect incomplete polymerizationso that thei polymer remains somewhat plastic. These strips aresubdivided into separate units, shearing them transversely at each ofthe spaces 13 (FIG. l). While these individual units may be extremelysmall (one-quarter inch by one-eighth inch in an example) thelaminations remain firmly adhered to each other and the Cil variouselectrical connections described remain undisturbed. Due to the pressingoperation that precedes the impregnation, the electrical connectionspreviously described are also preserved.

In FIG. 2 the united foil portions 34 and 36 which overlie dummy mica 18are normally covered by a fi'lm of the polymerized impregnant followingthe cutting operation. Nevertheless, wire leads 38 can readily besoldered to terminal portions 34 and 36 in the next operation withoutscraping the coating of impregnant `from the soldering areas. Finally, asuitable encapsulating coating such as an epoxy resin is applied tocomplete the unit.

The completed condenser (omitting the encapsulating covering) is shownin FIGURE 4. In this greatly enlarged view, it is apparent that Itheedges of electrodes 12 and 14 are separated by a limited length ofsurface path around the lateral edges of each mica. The impregnationgreatly enhances the voltage breakdown characteristics of this region;and the impregnation is carried out in a manner that does not disturbthe vital electrical connections to the film electrodes. Thestacked-mica impregnated units are seen to have -mechanically andelectrically strong terminals for soldering to wires 38, yet suchterminals are included without any extra fabricating operation, withouta separate operation of handling and mounting tiny metal end clips, andwithout any significant increase in bulk of the completed unit ascompared to such a unit lacking solderable foil terminals.

The foregoing detailed description includes a number of novel featuresand represents the presently preferred embodiment of the various aspectsof the invention. This description will naturally prompt those skilledin the art to make varied applications, substitutions and modificationsof those novel features, and the invention should be broadly construed,in accordance with its full spirit and scope.

Having thus described my invention what I claim as new and desire tosecure by Letters Patent is:

1. A mica condenser including a stack of rectangular -micas havingmetal-film electrodes bonded to opposite faces thereof and having theedges of the electro-des spaced inward of three edges of each mica,respectively, leadtin foils united to and extending from alternatefourth edge portions of respective opposed electrodes of successivemicas in the stack and around a respective edge of the stack and eachsaid lead-tin foil having a portion overlying an end portion of thestack, a pair of a metal that is relatively harder and of a relativelyhigher melting point than the lead-tin foils in the form of a channel ateach edge of the stack from which said lead-tin foils extend and havinga metallic bond thereto and overlying said end portions of the stack,`and one of said terminal foils embracing all of said micas and theother embracing. all but one of the micas, each of said terminal foilshav-- ing a terminal wire soldered thereto.

2. A mica condenser in accordance with claim 1 wherein said stackofmicas has a polymerized dielectric impregnant filling the voids thereof.

3. A mica condenser including a stack of micas having metal-filmelectrodes bonded to the opposite faces thereof, lead-tin foils unitedto confronting electrodes of `successive micas in the stack, said foilsextending about 0pposite edges of the stack from the spaces betweenalterna-te micas respectively and having portions overlying respectiveend portions of the stack, a pair of channelshaped terminal foils of ametal relatively harder and of relatively higher melting point than thelead-tin foils along said opposite edges of the stack each of saidterinitial foils embracing a plurality of said micas and at least one ofsaid terminal foils embracing all of said micas, the overlying portionsof said lead-tin foils having a metallic bond to each other and to saidterminal foils, and said lead-tin foils having a metallic bond to said 56 terminal foils and t-o respective ones of said metal-film ReferencesCited by the Examiner' electrodes remote from said overlying portions.UNITED STATES PATENTS 4. A mica condenser in accordance with claim 3,having a polymerized dielectric impregnant filling the spaces 31180951/1964 Baron 317-261 within said stack and having wire terminalssoldered to 5 FOREIGN PATENTS said overlying portions of said terminalfoils. 550,419 1/1943 Great Britain 5. A mica condenser in accordancewith claim 3 in- 582,651 11/ 1946 Great Britain,

cluding a wire terminal extending from each of said channel-shapedterminal foils and having fused-metal JOHN F' BURNS Prmary Examinerconnections thereto. 10 DARRELL L. CLAY, Examiner.

1. A MICA CONDENSER INCLUDING A STACK OF RECTANGULAR MICAS HAVINGMETAL-FILM ELECTRODES BONDED TO OPPOSITE FACES THEREOF AND HAVING THEEDGES OF THE ELECTRODES SPACED INWARD OF THREE EDGES OF EACH MICA,RESPECTIVELY, LEADTIN FOILS UNITED TO AND EXTENDING FROM ALTERNATEFOURTH EDGE PORTIONS OF RESPECTIVE OPPOSED ELECTRODES OF SUCCESSIVEMICAS IN THE STACK AND AROUND A RESPECTIVE EDGE OF THE STACK AND EACHSAID LEAD-TIN FOIL HAVING A PORTION OVERLYING AN ENDX PORTION OF THESTACK, A PAIR OF A METAL THAT IS RELATIVELY HARDER AND OF A RELATIVELYHIGHER MELTING POINT THAN THE LEAD-TIN FOILS IN THE FORM OF A CHANNEL ATEACH EDGE OF THE STACK FROM WHICH SAID LEAD-TIN FOILS EXTEND AND HAVINGA METALLIC BOND THERETO AND OVERLYING SAID END PORTIONS OF THE STACK,AND ONE OF SAID TERMINAL FOILS EMBRACING ALL OF SAID MICAS AND THE OTHEREMBRACING ALL BUT ONE OF THE MICAS, EACH OF SAID TERMINAL FOILS HAVING ATERMINAL WIRE SOLDERED THERETO.